1. Technical Field
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly to a tandem-type image forming apparatus.
2. Description of the Related Art
Known image forming apparatuses using an intermediate transfer method are generally equipped with a plurality of image bearing members arranged in tandem along a moving direction of an intermediate transfer member. Toner images are formed on the plurality of image bearing members and then transferred primarily onto the intermediate transfer member. Subsequently, the toner images are secondarily transferred onto a recording medium. In such image forming apparatuses using the intermediate transfer method, toner images are formed on at least two image bearing members and the toner images are primarily transferred onto the intermediate transfer member such that they are superimposed one atop the other, forming a composite toner image, which is then transferred onto a recording medium in a multi-color mode (first control mode). Alternatively, a toner image is formed on a single image bearing member, and primarily transferred onto the intermediate transfer member, and then transferred onto a recording medium in a single-color mode (second control mode).
In this type of image forming apparatuses, a developing agent deteriorates with time, causing insufficient charging of overall toner, which results in an image defects, such as roughness in a halftone image. To address this difficulty, in JP-2009-168925-A, a degree of degradation of an imaging device that forms the toner image on the image bearing member disposed at an extreme downstream end in the moving direction of the intermediate transfer member is detected. When the degree of deterioration reaches a certain level, a level of secondary transfer bias is reduced.
According to JP-2009-168925-A, in a case in which the charge amount of toner is generally low, flow of electrical charge due to movement of the toner is small at a secondary transfer portion, hence generating electric discharge at the secondary transfer portion and resulting in a rough image. In such a case, the secondary transfer bias is reduced so that generation of electric discharge at the secondary transfer portion is suppressed and the roughness in a low-density image is thus reduced.
In this type of image forming apparatus, that is, i.e., a tandem-type image forming apparatus, the plurality of image bearing members is disposed along the moving direction of a transfer medium such as a recording medium and the intermediate transfer member, and the toner images formed on the image bearing members are transferred onto the transfer medium. The toner images on the image bearing members are transferred onto the transfer medium by applying a transfer bias from a transfer device. A ratio (transfer rate) of toner constituting the toner image to be transferred onto the transfer medium depends on the charge amount of toner and the level of the transfer bias.
JP-H05-158357-A proposes a monochrome image forming apparatus that directly transfers a toner image from a single image bearing member, i.e., a photosensitive drum onto a recording medium (transfer medium). This image forming apparatus measures a number of printed sheets or a number of printed sheets corresponding to mixing of the developing agent. Based on the measurement, the transfer bias is adjusted to have an optimum transfer current which is inversely proportional to changes in the charge amount of toner with time. In this configuration, even when the charge amount of toner is generally low due to deterioration of the developing agent with time, adjustment of the transfer current according to the deterioration of the developing agent suppresses image defects attributed to the deterioration of the developing agent.
In the image forming apparatus using the intermediate transfer method, when the developing agent deteriorates with time, causing the charge amount of overall toner to drop low, primary transfer at the primary transfer portion may be affected. That is, even before secondary transfer, the toner image may be degraded. It is difficult to prevent such degradation of image quality only by adjusting the secondary transfer bias as described above. In order to suppress degradation of image quality attributed to the deterioration of the developing agent with time, causing the charge amount of overall toner to drop low, it may be necessary to adjust the primary transfer bias to be applied to the primary transfer portion.
FIG. 12 is a graph showing relations between a primary transfer rate and a primary transfer current in an initial state in which the developing agent has not deteriorated yet.
FIG. 13 is a graph showing relations between the primary transfer rate and the primary transfer current when the developing agent deteriorated with time.
FIGS. 12 and 13 show results of measurement on a 5% band image and a 95% band image. As illustrated in FIG. 14, the 5% band image is a band-shaped toner pattern having a width which corresponds to 5% of an entire imaging width, formed substantially in the center in a main scanning direction and extending in a sub-scanning direction (sheet moving direction). As illustrated in FIG. 15, the 95% band image is a band-shaped toner pattern having a width which corresponds to 95% of the entire imaging width, formed at one side in the main scanning direction, and extending in the sub-scanning direction (sheet moving direction).
In the initial state in which the developing agent has not yet deteriorated, the relations between the primary transfer rate and the primary transfer current for the 5% band image and the 95% band image look like the one shown in FIG. 12, in which there is a peak in the primary transfer rate for both images. An optimum level of the primary transfer current for achieving a highest possible primary transfer rate within a range in which substantially the same primary transfer rate is achieved for both the 5% band image and the 95% band image is similar to or the same value such as shown in FIG. 12. Generally, in most cases, the primary transfer current is set to the value shown in FIG. 12.
By contrast, when the developing agent deteriorated with time, the relations between the primary transfer rate and the primary transfer current for the 5% band image and the 95% band image look like the one shown in FIG. 13. In FIG. 13, the highest peak of the primary transfer rate for the 95% band image shifts largely toward a low primary transfer current (absolute value) side as compared with the initial state. In this case, the optimum primary transfer current for achieving the highest possible primary transfer rate within the range in which substantially the same primary transfer rate is achieved for both the 5% band image and the 95% band image is similar to or the same value such as shown in FIG. 13. As shown in FIG. 13, the absolute value of the optimum primary transfer current decreases with time as compared with the initial state.
The reason for the decrease in the optimum primary transfer current (absolute value) with the deteriorated developing agent with time is assumed as follows.
During which an amount of toner moving from the image bearing member to the intermediate transfer belt at the primary transfer portion increases with an increase in the primary transfer bias, flow of electric current caused by the movement of toner increases, hence increasing the primary transfer current. After the amount of movement of toner reaches a state of saturation, the flow of electric current caused by the movement of toner stops increasing. In this case, electrical discharge at the primary transfer portion increases in accordance with an increase in the primary transfer bias. Thus, once the amount of movement of toner reaches the state of saturation, the primary transfer current keeps increasing in accordance with generation of the electrical discharge.
On the other hand, with an increase in the electrical discharge, the primary transfer rate decreases. That is, after the amount of movement of toner reaches the state of saturation, the primary transfer rate decreases as the primary transfer current increases. As a result, as shown in FIGS. 12 and 13, there is a highest peak in the primary transfer rate in the relations between the primary transfer current and the primary transfer rate. When the developing agent deteriorates with time, the charge amount of toner is relatively low overall. In such a case, the flow of electric current caused by the movement of the toner at the primary transfer portion is less than that in the initial state so that the primary transfer current when the amount of move of toner reaches the state of saturation is less than that in the initial state. As a result, when the developing agent deteriorated with time, the optimum primary transfer current (absolute value) to achieve the optimum primary transfer rate is lower than that in the initial state.
However, the present inventors recognized that reducing uniformly the primary transfer current (absolute value) flowing through the primary transfer portion in accordance with the degree of deterioration of the developing agent may rather degrade the image quality. More specifically, as will be described later, although reducing the primary transfer current enhances the primary transfer rate, reducing the primary transfer current reduces the secondary transfer rate. Therefore, reducing the primary transfer current does not necessarily improve the image quality. Rather, it may reduce the image quality. It is also known that a rate of decrease in the secondary transfer rate after correction of the primary transfer current in the single-color mode (second control mode) is greater in the multi-color mode (first control mode). Thus, if the amount of correction of the primary transfer current in the multi-color mode (first control mode) is the same as or similar to that in the single-color mode (second control mode), the image quality is degraded more easily.
In the first control mode, the toner images formed on the plurality of image bearing members are transferred onto the intermediate transfer member such that they are superimposed one atop the other, forming a composite toner image. The composite toner image thus obtained is transferred secondarily from the intermediate transfer member to a recording medium. By contrast, in the second control mode in which only one of the image bearing members (hereinafter referred to as downstream image bearing member) used in the multi-color mode is used, one toner image without other toner images superimposed thereon is transferred secondarily from the intermediate member to the recording medium. As a result, an amount of toner to be transferred secondarily to the recording medium at the secondary transfer portion, in general, is greater in the first control mode than in the second control mode. Therefore, an optimum secondary transfer bias to achieve an optimum secondary transfer rate is greater in the first control mode than in the second control mode. Thus, the secondary transfer bias is greater in the first control mode than in the second control mode.
At this time, if the primary transfer bias is adjusted so that the primary transfer current is reduced in accordance with the degree of deterioration of the developing agent, the primary transfer rate is enhanced. However, since the charge amount of overall toner is relatively low due to deterioration of the developing agent and hence the primary transfer current is relatively small, the charge amount of toner at the secondary transfer portion is even lower than before correction. Degradation of image quality attributed to the decrease in the charge amount of toner at the secondary transfer portion is greater in the first control mode in which the secondary transfer bias is relatively high than in the second control mode in which the secondary transfer bias is relatively low.
FIG. 16 is a graph showing relations between the secondary transfer rate and the secondary transfer current associated with a toner image on the downstream image bearing member.
The relations between the secondary transfer rate and the secondary transfer current may be considered as having substantially the same relations as between the primary transfer current and the primary transfer rate. That is, during which the amount of toner moving from the image bearing member to the intermediate transfer belt at the secondary transfer portion increases with an increase in the secondary transfer bias, flow of electric current caused by the movement of toner increases, hence increasing the secondary transfer current. By contrast, after the amount of move of toner reaches a state of saturation, the flow of electric current caused by the movement of toner stops increasing. Consequently, the electrical discharge at the secondary transfer portion increases in accordance with an increase in the secondary transfer bias. In this case, the primary transfer current increases with an increase in the electrical discharge while the secondary transfer rate decreases with the increase in the electrical discharge. FIG. 16 shows the resulting relations between the secondary transfer current and the secondary transfer rate.
The secondary transfer current in the first control mode is set to achieve a highest possible secondary transfer rate within a range in which the secondary transfer rates for each of the plurality of toner images constituting the composite toner image are approximately the same (that is, none of the secondary transfer rates has a low value relative to all the other ratios). The toner image to be transferred primarily from the image bearing member disposed in the upstream side in the moving direction of the intermediate transfer member among the plurality of toner images constituting the composite toner image is charged up with the primary transfer current when passing through the primary transfer portion in the downstream therefrom. As a result, the charge amount of toner in the secondary transfer portion is higher than that of the toner image to be transferred primarily from the downstream image bearing member.
In a case in which the secondary transfer current for transferring secondarily the plurality of toner images having different charge amounts all at once is determined as described above, for the toner image with a relatively low charge amount (the toner image transferred primarily from the downstream image bearing member), the secondary transfer current is set to a value higher than a value (peak value) achieving the maximum secondary transfer rate. By contrast, in the second control mode using one image bearing member, i.e., the downstream image bearing member, because there is one toner image, the secondary transfer current is set to achieve the optimum secondary transfer rate for the toner image.
In a case in which the primary transfer bias is adjusted to reduce the primary transfer current for the downstream image bearing member in accordance with the rate of deterioration of the developing agent in the first control mode and in the second control mode in which the respective secondary transfer current is determined in a manner described above, the primary transfer current after correction is low for the toner image on the downstream image bearing member, resulting in a lower charge amount of toner in the secondary transfer portion than before correction. At this time, as the charge amount of toner in the secondary transfer portion decreases, the flow of electric current caused by the movement of the toner at the secondary transfer portion is reduced. Thus, the secondary transfer current when the amount of move of toner reaches the state of saturation (i.e., when the secondary transfer rate reaches its peak) is less than that before correction. As a result, the relations between the secondary transfer current and the secondary transfer rate after correction of the primary transfer bias indicated by a broken line in FIG. 16 shift towards the lower secondary transfer current side as compared with the relations before correction indicated by a solid line in FIG. 16.
As shown in FIG. 16, a rate of change in the secondary transfer rate relative to the change in the secondary transfer current tends to increase as the secondary transfer current shifts away from the peak value capable of achieving the maximum secondary transfer rate. The set value for the secondary transfer current in the first control mode before correction is at a higher secondary transfer current side than the peak value capable of achieving the maximum secondary transfer rate as described above. Consequently, when the peak value shifts toward a lower secondary transfer current side due to correction of the primary transfer bias, the set value for the secondary transfer current after correction shifts even further away from the peak value capable of achieving the maximum secondary transfer rate. As a result, the correction of the primary transfer bias causes the secondary transfer rate to drop significantly.
In the tandem-type image forming apparatus in which the toner images formed on the plurality of image bearing members are transferred onto a transfer medium such that they are superimposed one atop the other, preferably, the transfer current is corrected in accordance with parameters in correlation with the degree of deterioration of the developing agent such as the number of printed sheets as proposed in JP-05-158357-A.
The present inventors have recognized, however, that the effect of correction of the transfer current in accordance with the degree of deterioration of the developing agent relative to degradation of image quality differs depending on the toner images. Furthermore, in the single-drum type image forming apparatus in which the plurality of toner images is formed on the single image bearing member and transferred sequentially onto a transfer medium, when the transfer current is corrected in accordance with the degree of deterioration of the developing agent as described above, the effect of correction of the transfer current on degradation of image quality differs depending on the toner images.
The present inventors have also recognized that in the tandem-type image forming apparatus, the effect of correction of the transfer current in accordance with the degree of deterioration of the developing agent relative to the degradation of image quality differs depending on the image bearing members because the volume resistivity of toners in the developing agents used to form the toner images on the image bearing members differs. That is, depending on the volume resistivity of toners, the rate of change in the optimum value of the transfer current in accordance with the deterioration of the developing agent is different.
An apparent electrostatic capacity of toner having a relatively low volume resistivity decreases as the electrical resistivity decreases so that the toner is difficult to keep relatively the charge. When the charging ability of the toner decreases due to deterioration of the developing agent, the decrease in the charge amount of toner having the low volume resistivity is relatively large. As a result, even when the optimum transfer current is set corresponding to the volume resistivity of the respective toner in the initial state, with deterioration of the developing agent after extended use, the rate of change in the optimum transfer current from the initial state is greater in the toner with the low volume resistivity.
Therefore, if the same correction is performed on the transfer current in accordance with the degree of deterioration of the developing agent on the basis of the toner having a high volume resistivity, adequate correction is not performed for the toner with a low volume resistivity and hence degradation of image quality is not suppressed sufficiently. By contrast, if the same correction is performed on the transfer current in accordance with the degree of deterioration of the developing agent on the basis of the toner having a low volume resistivity, overcorrection occurs for the toner with a high volume resistivity and hence the transfer rate is not improved sufficiently, resulting in the degradation of image quality.